The medical device industry produces a wide variety of implantable electronic devices for treating patient medical conditions. For some medical conditions, medical devices provide the best, and sometimes the only, therapy to restore an individual to a more healthful condition and a fuller life. Examples of implantable medical devices designed to deliver therapeutic electrical stimulation include neurological stimulators, pacemakers and defibrillators.
One of the common benefits of implantable medical devices is that once the device has been surgically implanted, and the surgical incisions created in the patient's skin closed up, the device may function without requiring a physical interface with devices external to the patient. This creates compelling benefits to the patient by preventing the discomfort and risk of infection inherent in various articles projecting through cuts in the patient's skin. However, the inaccessibility inherent in a device that has been surgically implanted creates several challenges. Not the least of these challenges is providing power for active electrical componentry in the device.
The problem of providing electrical power to components contained in a hermetically sealed case implanted in a patient's body has been addressed to varying degrees of success in the preceding decades through advances in battery technology. In some implantable medical device applications the increased longevity and efficiency of modern batteries has been supplemented with the ability to recharge the battery via an inductive link. But the challenges and risks inherent in any effort to inductively transfer energy from a coil connected to an external charger to a coil connected to the battery of an implantable medical device through the skin and tissue of a patient have made applications of inductive recharging of implantable medical devices relatively uncommon in the industry.
The passing of energy through the skin and tissue of a patient to the implantable medical device may tend to result in some of the energy being dissipated in the form of heat in the patient's skin and tissue. Improving the inductive connection between the external charger and the implantable medical device by positioning the external coil in the closest possible proximity to the implantable medical device will help to minimize energy dissipation, but cannot eliminate it. Further, additional energy may be dissipated as heat through the case of the implantable medical device itself. This issue may tend to be exacerbated by the trend of steadily decreasing the size of medical devices, as a smaller case results in the increased density of the energy escaping, and thus greater heat. It is possible that a patient experiencing unchecked inductive energy transfer to their implantable medical device may experience significant discomfort and even potentially severe skin and tissue burns.
Thus, limitations are typically placed on the amount of energy that may be transferred per unit time to an implantable medical device. These limits are arrived at partially as a function of the size of the device and partially by incorporating an amount of presumed variance in the positioning of the external charger relative to the implantable medical device. Thus, even if the external charger is placed in a position that would result in comparatively little energy dissipation in the skin and tissue, the amount of energy that would be delivered by the external charger would still be limited by the assumed worst-case safety factors.
Efforts to empirically measure the amount of heat that is being generated in a patient's tissue during energy transfer have sometimes proven to be inaccurate and/or impractical. The mounting of a temperature sensor on the outside of the case of an implantable medical device has only limited utility because it measures only the temperature at that particular point and might not be sensitive to local hotspots and could be inconvenient by necessitating running a wire through the case.